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Abstract

We present detailed investigations of chromatic polarization effects, caused by fiber spools used in FDML lasers and buffering spools for rapidly wavelength swept lasers. We introduce a novel wavelength swept FDML laser source, specially tailored for polarization sensitive optical coherence tomography (OCT) which switches between two different linear polarization states separated by 45°, i.e. 90° on the Poincaré sphere. The polarization maintaining laser cavity itself generates a stable linear polarization state and uses an external buffering technique in order to provide alternating polarization states for successive wavelength sweeps. The design of the setup is based on a comprehensive analysis of the polarization output from FDML lasers, using a novel 150 MHz polarization analyzer. We investigate the fiber polarization properties related to swept source OCT for different fiber delay topologies and analyze the polarization state of different FDML laser sources.

Left: Cylindrical projection of the polarization state (θ, φ) into 2D. The graph shows 4 independent measurements, 3 of linearly polarized light (0°, 45° and 90°) as well as circularly polarized light. The light source was an FDML laser followed by bulk optical components to prepare the desired polarization states. Right: 36 SOP measurements of ASE prepared with a linear polarizer followed by a λ/4 plate generating various elliptic states. The linear polarizer was turned in 10° steps. The red dots indicate the measured polarization states on the Poincaré sphere.

Left: FDML laser in sigma ring configuration as used for measurements at 1550 nm. ISO: isolator, FFP-TF: fiber Fabry-Pérot tunable filter, PC: polarization controller, CIR: circulator, FRM: Faraday rotation mirror. The 1.89 km spool is made of dispersion shifted fiber with a zero dispersion wavelength around 1550 nm. Right: Output of laser L1 (polarization dependent SOA) at different polarization controller positions where good lasing occurs. First three plots with mirror instead of FRM, last plot with FRM in sigma ring. In each case, the output shows good linear polarization with the residual deviation being caused by birefringence in the fiber path between the SOA and the polarization analyzer.

SOP traces taken from output of laser L2 (polarization independent SOA) shows large polarization fluctuations inside the laser cavity: The polarization changes significantly throughout the sweep (130 nm around 1511 nm) and shows large sweep-to-sweep variation. Left: Single sweep recorded with the analyzer. Right: An average of 256 sweeps.

SOP after different delay spool implementations (D1...D3) for sweeps of 130 nm around 1310 nm. The traces show wavelength evolutions of the SOP measured across forward (colored dots) and backward sweep (black dots). Several wavelengths were annotated for the 90° curve of D1. The incident light was prepared with linear polarization oriented along 0°, 45° and 90° relative to the spool plane; the choices 0° and 90° are shown on the right bottom. The second spool (standing) is only used for delay spool implementation D4. D1: direct 2 km delay spool, D2: a sigma ring delay with a 1 km spool traversed two times and a Faraday rotation mirror, D3: like D2 but with a normal mirror instead of the Faraday mirror.

Left: SOP traces for sweeps of 140 nm around 1550 nm when adjusting polarization paddle controllers after a 460 m fiber delay line. Paddles alter the location on the Poincaé sphere but have little influence on the extent of the traces. Right: Projection onto the gain axis for an SOA that amplifies only linear polarization with φ = 0°. For any incoming polarizations, the isolines in the graph represent the fraction of the incoming light which will be amplified in the SOA.